| Internet-Draft | Dynamic MultiPath Routing | November 2021 |
| Pfeifer & Widmann | Expires 31 May 2022 | [Page] |
Dynamic MultiPath Routing (DMPR) is a loop free path vector routing protocol with built-in support for policy based multipath routing. It has been designed from scratch to work at both low and high bandwidth networks - even with high packet loss. The objective was to keep routing overhead low and ensure a deterministic protocol exchange behavior. DMPR can be used to manage larger networks with characteristics based on BGPv4 with transport and self-configuration properties taken from OSPF/OLSR. Unlike BGPv4 or OSPF, DMPR does not support higher network separation concepts. A DMPR network is a flat network in which DMPR nodes have equal tasks. This also applies to DMPR communication. Unlike OLSR/OSPF there is no flooding messages (topology broadcast), information are stored, accumulated/compressed and forwarded at each DMPR node. This feature contributes to the message load being deterministic.¶
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Todays mobile wireless networks have a diversity of requirements on the wireless links. To meet these requirements, it is possible to attach multiple network access technologies on the router and select, depending on the CoS of the packet, over which wireless link the packet is sent. This is the main idea of policy based multipath routing. The established routing protocols do not support the use of multiple access technologies on a single router. To tackle this issues, DMPR as been developed as a protocol for policy based multipath routing in and between mobile networks, which consist of multiple wireless links with different characteristics. DMPR makes it possible to calculate multiple routing tables and maintain the best paths for multiple policies.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].¶
The following list describes the terminology used in this RFC¶
Section 2 describes the information every node has and gets and how it should handle this information. Section 3 describes the behavior of the protocol in different scenarios and how it achieves particular features. Section 5 describes the message format in detail. Section 6 describes optional features that can be implemented to improve or extend DMPR but are not required for the basic functionality of propagating routing information.¶
Appendix A has a few policy examples. Appendix B lists all constants used in this RFC where some of them are configurable. Appendix C shows a few simple examples of how DMPR behaves under specific network conditions.¶
Every router periodically sends out unicast or multicast messages to all routing enabled links. This message already includes routers database (best path to each node) known by this router. A router receiving this message adds itself to all these paths, chooses the best path to each node among all those it got from its neighbors and advertises these paths itself via unicast or multicast messages (if received as Unicast, it MUST be returned as unicast). This is standard procedure in a path-vector routing protocol. In DMPR the paths are further separated by a policy, therefore it is possible to have more than one path to each node. Also included in the message are all attributes of this path so that the receiving node can make informed decisions on whether this path is the best under the current policy. In the end there exist multiple paths through the network and packets can be routed according to their requirements which for example can be the path with the highest bandwidth or with the lowest latency.¶
Traditionally, routing protocols find the best path using a scalar metric. This metric may be a simple constant stating the preference of the link or may be a computed metric using several factors such as bandwidth, latency or cost. Furthermore, this metric is only known locally or, for example in the case of BGP-4, where external paths can be marked with a local preference for internal peers, to a small subset of the network. In DMPR a policy is a globally defined function that defines a function how to determinate the best path. For this reason, the policy has all link attributes of each path at its disposal and therefore has a higher control over which path it chooses.¶
To maintain the routing data, all data extracted from the received routing messages is organized in different data structures. These data structures are used for calculating the routes and creating of the routing tables.¶
Every message received by a node is saved in the Message Information Base. This data structure is the basis for all subsequently emitted routing messages and the routing table. The Message Information Base contains the latest (by sequence number) received message for each neighbor. Each entry is controlled by a hold timer and is purged after the hold timer expires. This timer is reset as soon as a new valid message from this neighbor is received. This data structure contains potentially duplicate or conflicting data since many neighbors send their paths and subnet information for all nodes they know of. This is intended and required because any message may be deleted at almost any time when its hold timer expires.¶
The Routing Data is computed from the Message Information Base. In contrast to the Message Information Base this is data structure contains no conflicting information and can be seen as the single source of truth for the routing table and routing message generation. It contains the best path to each known node grouped by policy. Furthermore the advertised subnets from each node are combined while following the network retraction rules (see Section 3.9).¶
The routing table contains the best path to each known subnet grouped by policy. This table is meant to be inserted into the underlying operating system's routing table.¶
DMPR supports unicast and multicast neighbor detection and transport schemes. The scheme MUST be configurable at link level (e.g. eth0: multicast, eth1: unicast). Multicast provides ad-hoc capabilities without prior knowledge of neighbor nods. The multicast detection mechanism and transport mechanism are similar to those of other ad-hoc routing protocols such as OLSR.¶
Unicast detection and transport lacks support of ad-hoc configuration: the neighbor list must be configured a prior. The advantage of using unicast is that DMPR can also be used on networks that do not or insufficiently support multicast. Notes: an alternative for the use of unicast is the use of tunnels (IPIP, GRE, ...). For example, the tunnel is the preferred solution when BGAN terminals or routing foreign segments must be bridged.¶
DMPR is build around the concept of identifying nodes uniquely via DMPR ID. It is therefore irrelevant whether neighbours are detected several times via different links and unicast or multicast.¶
Each node listens to a unicast or multicast address at each enabled DMPR link. If multicast, the multicast address SHOULD be configurable. Periodically, after a defined message interval + jitter, a node sends out its routing message, which includes:¶
When a node receives a message, it deduces a connection and therefore a path to the sender via the interface that message came in. With this mechanism the path to a node propagates through the network. Every received message SHOULD be assigned to a hold timer. When this hold timer expires, the message MUST be deleted from the Message Information Base. The timeout SHOULD be a multiple of the routing message interval. Asymmetric links are handled with a feature called reflection, which is described in Section 5.4.¶
For unicast, DMPR MUST support a list of IPv4/IPv6 addresses of DMPR neighbors at a particular link. The messages and timeout constraints are identical to previous multicast section.¶
Whenever a neighbor is not reachable anymore (e.g. due to topology change), no further routing messages will be received from this neighbor. As all the received messages are assigned to a timer, no routing messages of the lost neighbor will be present in the Message Information Base, after the expiration of this timer. This means, that the loss of the neighbor is detected after the timeout of the last received routing message of the concerned neighbor. For this reason, the routing table SHOLD be recalculated, whenever the Message Information Base is purged due to timeouts.¶
In DMPR all interfaces that are registered with the DMPR daemon are treated completely separate from each other. Routing messages are sent over each one individually. This ensures that all possibly accessible neighbors are reachable. The message information base (seeSection 2.1) is grouped by interface. Therefore nodes can detect links on each interface individually.¶
Each message received by a neighbor is saved in the Message Information Base (seeSection 2.1). With this message a hold timer is set that purges the messages after the given time. To improve the message size a node can choose to only send partial updates (i.e. differential, only the changes since the last full update). To support this a node has to retain the a version from the last full message so it can apply the new partial message.¶
To support routing different traffic types over different routes, DMPR supports multiple policies. A policy defines, how the best path to a destination is computed using the available link attributes. For all the defined policies, seperate routes will be calculated to every reachable destination. The best path to all destinations is included in the routing message for every policy. For each policy, a seperate routing table MUST be generated. In large networks, this results in a multi topology routing. When different parts of the network have different attributes (e.g. one path has a low loss rate, another path in contrast has a higher bandwidth), different subsets of the topology will be used to forward packets that require different policies (Class of Service).¶
DMPR MUST know the link attributes that are required to determinate the best path for all the registered policies on all links (interfaces) to the router. These attributes can either be configured staticly by the network administrator or can be dynamically gathered from the attached modem devices. This RFC does not specify how the link information has to be gained. There is no specification defining which attributes must be given to the protocol (e.g bandwidth, loss rate, latency). The requirement of attributes depends on the policies meant to be used.¶
The calculation of the best route MUST be defined for each policy separately. For example simple policies only consider a single link metric (such as bandwidth or loss rate) for the calculation of the best route. Combined policies might use a combination of multiple link metrics. The route selection mechanism is part of a policy's definition and therefore can be individually defined.¶
For every reachable destination, the routing data is calculated for all defined policies using the policy's specific route selection mechanism. As a result, there MUST be a seperate routing table for every defined policy. Whenever new crucial information is received, the routing data MUST be recalculated. Information that causes a recalculation of the routing data can be:¶
Each network advertised in a message has an optional flag called "retracted". This flag is set to true when a node no longer advertises this network as available. The only node to ever set this flag to true MUST be the originally advertising node. A set retracted flag always supersedes an unset flag. Networks are forwarded with this ruleset:¶
| Network known as not retracted | Network known as retracted | Network set as retracted in a message | Action to take |
|---|---|---|---|
| false | false | false | Insert network in known networks and forward |
| false | false | true | ignore network, do not forward |
| false | true | false | forward network as retracted |
| false | true | true | forward network as retracted |
| true | false | false | just forward network |
| true | false | true | set network in known networks as retracted and forward retracted |
| true | true | false | illegal |
| true | true | true | illegal |
The parameters described in this section SHOULD be configured at the startup of the router. This specification does not define any values for the parameters.¶
Core Configuration describes the fields which MUST be the same on all routers in the neworks.¶
This field describes a list with all the policies which should be considered by the routing protocol. The policies are referenced by name. For every listed policy, the routing protocol MUST implement the according route selection mechanism.¶
This field describes a list with all the routers physical interfaces, which are used as DMPR interfaces. For all interfaces following fields are required:¶
A DMPR packet consists of a preamble, followed by zero or more extension headers followed by zero or one payload. Each extension header and payload is defined by a type.¶
| Type | Use |
|---|---|
| 0-119 | Extension Header |
| 120-127 | Extension Header, reserved for private use |
| 128-247 | Payload |
| 248-255 | Payload, reserved for private use |
Possible Types are defined in further detail below¶
The preamble of a DMPR packet is as follows¶
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Magic| Reserved| NextType | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+¶
The preamble of a packet¶
An extension header consists of the type immediately following this header, a length specifier, and the Extension Header data.¶
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NextType | Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | + Data + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+¶
The Payload consists of the data from the end of the preamble or last extension header until the end of the packet. Payloads may be recursive, i.e. contain a valid packet (or parts of it) in themselves. Payload processors therefore MUST have the ability to feed their result back into the message processing chain. This behavior is defined by the payload itself.¶
Type: 127¶
This is a keep-alive packet, the payload length is zero. Implementations SHOULD reset the message hold timer for the sending node upon receiving a keep-alive packet¶
Type: 128¶
Unkompressed, plain, standard-compliant I-JSON data as described in [RFC7493]. This is the main routing data; its structure is defined in Section 5.2¶
Type: 129¶
LZMA-compressed standard-compliant I-JSON data as described in [RFC7493]. This is the main routing data; its structure is defined in Section 5.2¶
Type: 130¶
A packet greater than the MTU between two nodes SHOULD be fragmented using the fragmentation payload.¶
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identifier |L|Packet offset| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Payload | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+¶
The Fragmentation Payload Header¶
When a packet is larger than the MTU of the link between two nodes it SHOULD be fragmented. For this purpose, the sending node computes the maximum effective payload size for packets sent (i.e MTU less preamble, possibly extension headers and the fragmentation header) and splits the original packet into parts with this size. For each of these parts, it sends a packet with the fragmentation header set to a common identifier, a corresponding packet offset and the LAST bit set for the last fragment.¶
The receiving node keeps track of all received fragments, grouping them by source address and identifier. As soon as all fragments of a packet have been received, the reconstructed packet MUST be fed back into the message processing chain as if it were a new, just received packet. Fragments MUST be regularly purged based on a hold timer.¶
A JSON payload is an ASCII-7 encoded JSON object. A sending node SHOULD use ascii-encoding for the JSON data. A receiving node MUST be able to decode UTF-8 encoded data. The payload can be zip compressed. A compressed payload has to be announced in the header.¶
Augmented requirements language for this section:¶
General Message Structure¶
{
"id": <NODE_ID>,
"seq": <SEQUENCE_NUMBER>,
"type": <TYPE>,
"partial-base": <SEQUENCE_NUMBER>,
"addr-v4": <IPv4_ADDRESS>,
"addr-v6": <IPv6_ADDRESS>,
"networks": {
<IPvX_NETWORK>: {},
<IPvX_NETWORK>: {
"retracted": true
}
},
"routing-data": {
<POLICY>: {
<NODE_ID>: {
"path": <PATH>
}
}
},
"node-data": {
<NODE_ID>: {
"networks": {
<IPvX_NETWORK>: {},
<IPvX_NETWORK>: {
"retracted": true
}
}
}
},
"link-attributes": {
<LINK_ATTRIBUTE_ID>: {
<LINK_ATTRIBUTE>: <METRIC>
}
},
"request-full": union(true, [<NODE_ID>, ...]),
"reflect": {
<REFLECT_DATA>: <DATA>
}
"reflected": {
<NODE_ID>: {
<REFLECT_DATA>: <DATA>
}
}
}
¶
Key and value description:¶
object: A path to each reachable node for each policy. POLICY is the name of a policy defined in the sending node. If the receiving node does not understand this policy the entry MUST be ignored. PATH: a path to a node described according to this syntax:¶
path = node [node-id ">[" link-attribute-id "]>" path] node-id = *ALPHA link-attribute-id = *DIGIT¶
Each message has a type. This RFC describes two types, namely full and partial, which are described in further detail here.¶
A full update SHOULD replace all data from the sending node in the receivers Message Information Base. It MUST NOT require any previous knowledge of the sender by the receiver. The following keys are specified:¶
A partial update only replaces the changed data in the receivers message information base. It therefore has the additional field "partial-base" which describes the sequence number of the base message, which MUST be a full update message, to which the changes apply. Note: A partial update only describes changes to a previous full update, never to a previous partial update. If the receiving node is unable to apply the partial update, e.g because it lacks the base message, then this node SHOULD use the "request-full" procedure to request a new full update (seeSection 5.3).¶
The following keys are specified:¶
When a node is not able to apply a partial update or just joined a network, it SHOULD send out a request for a full update using the request-full key in the message. This key may be an array containing NODE_IDs from which a full message is needed or may be the Boolean value true, to indicate that a full message from every neighbor is required. When a node receives a request-full key in a message that either has the value true or its ID present in the array, it MUST do one of the following: variant1: schedule the next message it sents to be a full message. variant2: send a full update immediately and reset its message interval timer, except when the last message already was a out-of-band full message, in which case it MUST/SHOULD schedule the next message according to the message interval timer to be a full message.¶
Reflections are a extensible mechanism and allow a node to exchange data with neighboring nodes, with 2-hop neighbors and with itself. When a node includes arbitrary JSON data in the reflect key in its message, each node receiving this message MUST send this data in the reflected key of its messages under the corresponding NODE_ID. A node MUST support reflecting all requests but is not required to actually parse that data. Because the messages are sent to all neighbors, every 2-hop neighbor becomes aware of the reflected data of a node. This fact is not used in this RFC but may be used in extensions of this protocol.¶
How asymmetric link detection should work/how it can be implemented¶
When/how to switch the mode¶
The original specification of xml2rfc format is in RFC 2629 [RFC2629].¶
Bidirectional Forwarding Detection CAN be used to detect link abortions. DMPR is designed to reduce the network usage to a minimum. Operating additionally BFD may increase the load unnecessarily.¶
An alternative is to increase the transmission interval of DMPR directly. The advantage is that no "context free" BFD packet are transmitted additionally, but in-line DMPR packets which reduce the convergence time in the entire network. BFD would have advantages if the DMPR interval is in the higher minute range, but here it would have to be considered whether to reduce the interval and do without BFD.¶
This template was derived from an initial version written by Pekka Savola and contributed by him to the xml2rfc project.¶
This memo includes no request to IANA.¶
All drafts are required to have an IANA considerations section (see Guidelines for Writing an IANA Considerations Section in RFCs [RFC5226] for a guide). If the draft does not require IANA to do anything, the section contains an explicit statement that this is the case (as above). If there are no requirements for IANA, the section will be removed during conversion into an RFC by the RFC Editor.¶
All drafts are required to have a security considerations section. See RFC 3552 [RFC3552] for a guide. Due to the Multicast Transmission, the routing message's payload data is unencrypted.¶
many exchangeable policies¶
use path with lowest overall loss. Overall loss is the accumulation of the loss rates of all hops to a destination. Required link attributes:¶
use path with highest bandwidth In multihop topologies, the overall bandwith is the minimum of the bandwidths between the hops to a destination. Required link attributes:¶
The different magic values and what they affect, how they could/should be set¶
Examples¶